The present invention relates generally to the field of integrated circuit devices and, more particularly, to the fields of integrated circuit semiconductor devices and Integrated Optical Circuits (IOC).
Integrated Optical Circuits (IOC) have received considerable attention in the literature due to their potential for high speed signal handling and signal processing capabilities. The basic functions of IOC are to control and detect guided light. Integrated optical devices are typically fabricated in materials that exhibit special optical properties such as photoconductivity, electrooptic or acoustooptic effects. Dielectric waveguides, which are the fundamental component of most IOC devices, consist of regions of high index of refraction surrounded by regions of lower index of refraction, thereby propagating light which wave launched down the waveguide.
Each IOC function can be performed best in a material specifically suited for that function. For example, lithium niobate has exhibited modulation bandwidths in excess of 5 GHz, and has been used in an optical spectrum analyzer, and in a one-dimensional spatial light modulator. Low loss waveguides have been fabricated in lithium tantalate and complex IOC functions have been demonstrated in lithium niobate such as an optically activated half adder which used channel waveguide modulators and cadmium sulfide films. This circuit demonstrated that complex functions can be performed by IOC, and would be of significant value if the potential speed of the electrooptic material could be realized by the entire circuit. However, the potentially high speed operation of the electrooptic materials has not been fully utilized due to the difficulty of interfacing between the optical regime and the electronic regime. That is, the integrated detectors were much slower than the modulators used in the IOC devices.
Schemes for detection of light in LiNbO.sub.3 substrates have been demonstrated, and they fall into two broad categories: discrete detectors that must be attached to the IOC, and integrated detectors. Discrete detectors are often high performance silicon photodetectors, (e.g. avalanche photodiode or p-i-n photodiode), but the discrete nature of the components makes the alignment of the electrooptic substrate to the photodetector cumbersome and difficult and the optical coupling obtained is not maximized. Furthermore, the combined structure has potential reliability problems and is not suitable for mass production. Integrated structures have been made using photoconductive materials that could be deposited onto the substrate, such as polycrystalline cadmium sulfide (CdS). Although CdS has a large light-to-dark resistance ratio, it is limited to operation below about 20 KHz. This has been sufficient to demonstrate many IOC operations, but is not fast enough to be of much practical value.